1. Field of the Invention
The present invention relates to intercalated clay compositions which have useful catalytic and adsorbent properties. More particularly, the invention relates to clay whose interlayers have been intercalated with silica.
2. Description of the Prior Art
Layered naturally occurring and synthetic smectites such as bentonite, montmorillonites and chlorites may be visualized as a "sandwich" composed of two outer layers of silicon tetrahedra and an inner layer of alumina octahedra. These "sandwiches" or platelets are stacked one upon the other to yield a clay particle. Normally this arrangement yields a repeating structure about every nine angstroms. A considerable amount of study has shown that these platelets can be separated further, by as much as 30 to 40 A, i.e. interlayered by intercalation of various polar molecules such as water, ethylene glycol, and various amines. The interlayered clays thus far prepared from naturally occurring smectites, however, are not suitable for general adsorbent and catalytic applications because they tend to collapse when subjected to high temperature.
In the past, clay materials have been intercalated with a variety of materials in order to form a supported open structure material which is useful as an adsorbent, a catalyst support, filtration medium or the like. However, it has been difficult to obtain a modified clay material which is stable at relatively high temperatures on the order of 250.degree. C.-500.degree. C. When a solvent swollen clay is heated to high temperature, the solvent is vaporized and collapse of the silicate sheets of the clay results as the solvent is removed from the interlamellar regions. This collapse significantly reduces the surface area of the clay because the internal surfaces are no longer available for adsorption. To solve this problem, a number of approaches have been taken to modify the clay by introducing supports or "columns" of material into the interlamellar regions of the clay to hold the silicate sheets of the clay apart. For instance, in the preparation of clay materials intended for use at relatively low to moderate temperatures, the clay layers have been separated with an organic material. For instance Schabtai et al, Proc. 6th Int. Congr. Catal., B5, 1-7 (1976) show a system in which smectite is interacted with di-or polycations derived from rigid, preferably cage-like amines, which acquire a single stable orientation in the interlayer space because of the steric requirements dictated by the configuration of their structure. A 1,4-diazabicyclo (2,2,2) octane-montmorillonite was found to posses significant molecular sieve properties and markedly higher catalytic activity for esterification of carboxylic acids in comparision to ordinary alkylammonium-exchanged montmorillonites. Seto et al U.S. Pat. No. 4,159,994 show the intercalation of kaolin materials with an ammonium salt of a carboxylic acid having more than two carbon atoms, the alkali metal salt of a carboxylic acid having more than two carbon atoms, a lower alkylene glycol or a quaternized ammonium radical.
Because of the failure of the organic material impregnated clays at high temperatures, approaches have been taken to improve the stability of intercalated clays at high temperatures by intercalating clay substrates with various metal compounds. Thus, Lahav et al, Clays and Clay Minerals, 26, 107-115 (1978) have demonstrated the intercalation of smectite with interlamellar structures derived from aluminum, iron and nickel hydroxides. Yamanaka et al, Clay and Clay Minerals, 27(2), 119-124 (1979) have shown the intercalation of Na-montmorillonite with zirconyl chloride which gives rise upon dehydroxylation to internal pillars of zirconium oxide. This system appears to give rise to a structure which is thermally stable at elevated temperatures. Yet another metal salt intercalated clay system has been shown by V. E. Berkheiser et al in Clay and Clay Minerals, 25, 105-112 (1977) where smectite is intercalated with 1,10 phenanthroline metal complexes. Traynor et al, Clay and Clay Minerals, 26, 318-326 (1978) have shown the intercalation of smectities with bipyridyl metal complexes. Still further Loeppert et al, Clay and Clay Minerals, 27(3), 201-208 (1979) have demonstrated the intercalation of aqueous dispersions of Na.sup.+ -smectite or n-butylammonium-vermiculite with sulfate salts of Fe (II), Co(II) or Ni(II) bipyridyl or 1,10-phenanthroline complexes, whereby intercalated phases with spacings of about 29.5 A are obtained. Still another disclosure of the stabilization of clay materials in U.S. Pat. No. 4,176,090, wherein the porous interlayer structure of a clay is stabilized with "pillars" of stable inorganic polymers of oxides of metals such as aluminum, zirconium and/or titanium. The oxide pillars are formed by hydrolysis of salts of the metals which have been impregnated into the open structure of the clay at the appropriate pH. While the prior art shows examples of clay intercalated with metal oxide structures which impart significant high temperature stability to the clay, nevertheless the prior art does not disclose clay substrates intercalated with silica which is a substance expecially well known for its high temperature stability characteristics. In this regard De Le Cruz et al, Proceedings of the International Clay Conference, 1972 (Madrid, Spain, June 23-30), pp. 705-710 have shown modified montmorillonite and vermiculite substrates which have been treated with trimethylchlorosilane or dimethyldichlorosilane, with the indication given that the organochlorosilanes react with interstitial water in the clay substrate treated to form silanol groups therein. However, hydrolysis of these organosilane reactants does not result in the formation of silica or hydrated silica structures in the clay. Moreover, it has been postulated that if the organosilanol impregnated clay is heated, the silanol compounds would react to form low molecular weight siloxane compounds which are not attached to the interlayer frameword of the clay and could be eliminated from the clay substrate. A need therefore continues to exist for clay intercalated with silica structures.